How do you determine if the thermostat is working properly

You could have an issue with the thermostat being stuck open. This will cause a check engine light because the engine doesn't get to operating temp as quickly as programmed. A good scanner can watch live data to monitor this. The fuel cap leak could be a bad fuel cap or a leak in the evap system. Both issues need to be looked at by a technician. If you don't work on your own car then use this link to find a shop near you. https://repairpal.com/repair-shops Alex